Model Proliferation / Ability to set table name at runtime
date | some_foreign_key | total

activity_by_foo
activity_by_bar
activity_by_baz

app/models/activity_by_foo.rb
app/models/activity_by_bar.rb
app/models/activity_by_baz.rb

class Activity < ActiveRecord::Base
  def self.totals_for_period(start_date, end_date, id)
     select(:total).where(:start_date => start_date, :end_date => end_date,  :some_foreign_key => id).sum(:total)
  end
end